Temperature switch

ABSTRACT

A temperature switch comprises a material ( 14 ) which melts and concomitantly undergoes a change in an electrical characteristic at a predetermined temperature. The temperature switch also includes at least two electrical contacts ( 10, 12 ) for detecting the change in the electrical characteristic. The material ( 14 ) comprises an organic cation and an anion and is preferably an ionic liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of GB application Serial No.0521287.3, filed Oct. 19, 2005, which application is incorporated hereinby reference.

TECHNICAL FIELD

The invention relates to a temperature switch.

BACKGROUND

A known temperature switch comprises an electrically conductive sheathand an electrically conductive wire positioned within the sheath so asto be spaced from (that is to say not in contact with) the sheath. Asolid eutectic salt mixture lies between the sheath and the wire and isin contact with both the sheath and the wire. The eutectic salt mixtureis selected so as to melt at a desired threshold temperature. Attemperatures below the threshold temperature, the solid eutectic saltmixture is poorly electrically conductive (it has a high electricalimpedance). However, on melting, the eutectic salt mixture undergoes alarge increase in electrical conductivity (that is to say its electricalimpedance decreases). In use, the conductive sheath and the conductivewire act as electrical contacts for detecting the change inconductivity/impedance of the eutectic salt mixture which lies betweenthe sheath and the wire. The change in conductivity/impedance isdetected by suitable electronics connected to the sheath and the wire.

SUMMARY

In accordance with the invention, there is provided a temperature switchcomprising a material which melts and concomitantly undergoes a changein an electrical characteristic at a predetermined temperature, and atleast two electrical contacts for detecting the change in the electricalcharacteristic, the material comprising an organic cation and an anion.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a more detailed description, by way of example only, oftemperature switches in accordance with the invention, reference beingmade to the appended drawings in which:

FIG. 1 is a schematic view of a temperature switch and;

FIG. 2 is a graph showing an idealised change in impedance on melting ofan ionic liquid used in the temperature switch of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, the temperature switch comprises an electricallyconductive sheath 10, an electrically conductive wire 12 and an ionicliquid 14. As seen in FIG. 1, the electrically conductive wire 12 passescentrally through the conductive sheath 10. The conductive wire 12 isspaced from, and so does not touch, the conductive sheath 10. In orderto ensure that the conductive wire 12 does not contact the conductivesheath 10, a porous spacing material (not shown) may be provided aroundthe conductive wire 12.

As used herein, the term “ionic liquid” refers to a salt consisting ofan organic cation and an anion. The anion may be either inorganic ororganic. Despite the name, ionic liquids are not necessarily liquids atroom temperature. However, they may have relatively low melting pointscompared to inorganic salts. For example an ionic liquid may have amelting point less than 200° C. Also, at least one of the component ionsof the ionic liquid may have a delocalised charge. The ionic liquid 14shown in FIG. 1 is contained within the conductive sheath 10 and liesbetween the conductive wire 12 and the conductive sheath 10. The ionicliquid 14 is in contact with both the conductive wire 12 and theconductive sheath 10 and provides a continuous electrical pathwaybetween the wire 12 and the sheath 10. The ionic liquid 14 has a meltingpoint at a threshold temperature which is higher than the temperature towhich the temperature switch is normally exposed. Below the thresholdtemperature, the ionic liquid 14 exists in a solid state and has arelatively low electrical conductivity (a high impedance). Above thethreshold temperature, the ionic liquid 14 exists in a liquid state andhas a relatively high electrical conductivity (low impedance). If aporous spacing material is provided around the conductive wire 12 asdescribed above, then the ionic liquid 14 fills the pores of the spacingmaterial so as to provide a continuous electrical path between theconductive wire 12 and the conductive sheath 10.

There are a very large number of known ionic liquids. Many of these maybe capable of being used in the temperature switch, although some aremore preferred than others. As already stated, an ionic liquid consistsof an organic cation and an anion. Examples of suitable cations andanions are disclosed in detail in WO03/058185 and U.S. Pat. No.5,827,602.

Examples of preferred cations for the ionic liquid 14 are: imidazolium;substituted forms of imidazolium; pyridinium; substituted forms ofpyridinium; pyrrolidinium; substituted forms of pyrrolidinium;phosphonium; substituted forms of phosphonium; ammonium; substitutedforms of ammonium; guanidinium; substituted forms of guanidinium;uronium; substituted forms of uronium; thiouronium; and substitutedforms of thiouronium. A substituted form of phosphonium which isparticularly preferred is tetraalkyl phosphonium. A substituted form ofammonium which is particularly preferred is tetraalkyl ammonium.

The substituted cations may have, by way of example only, in place ofhydrogen, one or more of the following substituent groups: fluorine;alkyl groups of one or more carbon atoms; alkylene groups of two or morecarbon atoms; phenyl groups; and alkoxy groups. Alkyl, alkylene, phenyland alkoxy substituents may themselves be substituted with, by way ofexample, one or more of the following electron withdrawing groups: F—;Cl—; CF₃—; C₂F₅—; CF₃S—; (CF₃)₂CHS—; and (CF₃)₃CS—.

Preferred anions for the ionic liquid 14 are: sulphate; substitutedforms of sulphate; sulphonate; substituted forms of sulphonate; halides;amide; substituted forms of amide; imide; substituted forms of imide;tosylate; substituted forms of tosylate; borate; substituted forms ofborate; phosphate; substituted forms of phosphate; antimonate;substituted forms of antimonate; carboxylate; and substituted forms ofcarboxylate. A preferred substituted form of sulphate is alkylsulphate.

The substituted polyatomic anions, e.g. tosylates, phosphates, sulphatesetc. may, by way of example only, have, in place of hydrogen, one ormore of the following substituent groups: fluorine; alkyl groups of oneor more carbon atoms; alkylene groups of two or more carbon atoms;phenyl groups; and alkoxy groups. The alkyl, alkylene, phenyl and alkoxygroups may themselves be halogenated.

Any ionic liquid, including any combination of a preferred cation fromthe list given above with a preferred anion from the list given above,is a potential candidate for the ionic liquid 14 of the temperatureswitch. However, the ionic liquid chosen for the temperature switchshould, of course, have a melting point at a temperature which issuitable for the application in mind. For many applications, thetemperature switch will be used to provide a warning signal when thetemperature to which the temperature switch is exposed reaches orexceeds a predetermined threshold temperature. For any such applicationan ionic liquid should be chosen which has a melting point at or closeto the desired threshold temperature. For many applications, thetemperature switch preferably has an ionic liquid 14 with a meltingpoint below 200° C. and more preferably at or below 120° C. Meltingpoints in the range of from 80° C. to 120° C. are particularly preferredfor some applications.

In addition to an appropriate melting point, there are a number ofcharacteristics which are preferred characteristics of the ionic liquid14 of the temperature switch.

Firstly, the ionic liquid 14 preferably has a well defined, sharpmelting point. The presence of this characteristic can be readilydetermined by trial and error. A sharp melting point is favoured by acrystalline, ionic solid state and high purity.

As indicated above, the conductivity of the ionic liquid 14 increases(the impedance decreases) as the ionic liquid 14 melts. FIG. 2 shows adesirable impedance profile of an ionic liquid as the ionic liquidmelts. Preferably, as for the example given in FIG. 2, the decrease inimpedance exhibited by the ionic liquid 14 at the melting point 16 is ofconsiderable magnitude. A relatively high impedance in the solid stateis desirable. Also, it is desirable for the decrease in impedance tooccur as sharply as possible at the melting point. The impedance profilecan be readily measured so as to assist in the selection of a preferredionic liquid.

Another desirable feature of the ionic liquid is a relatively lowviscosity when melted. Again viscosity can be easily measured so as toassist in the selection of a preferred ionic liquid. Low viscosity maybe favoured by fluorinated anions such as hexafluorophosphate andtetrafluoroborate.

Further, the ionic liquid 14 should preferably be hydrophobic. To someextent, the hydrophobicity can be determined by selecting a suitableanion. For example, a halide anion tends to give an ionic liquid whichhas low hydrophobicity (i.e. has a high degree of water miscibility).When the anion is borate or a substituted form of borate, thehydrophobicity of the ionic liquid tends to be greater. Ionic liquids inwhich the anion is phosphate or a substituted form of phosphate are evenmore preferred, as these tend to give even greater degrees ofhydrophobicity. Fluorinated borates and phosphates, such astetrafluoroborate and hexafluorophosphate are particularly preferredanions as they give ionic liquids with very high hydrophobicities.

Preferred ionic liquids are also chemically and thermally stable. Inparticular, they should be thermally stable to high temperatures.Preferably, they are thermally stable to at least 300° C. Ionic liquidsin which the anion is tris-(pentafluoroethyl) trifluorophosphate areparticularly preferred as this anion gives both high levels ofhydrophobicity and also chemical stability.

Finally, preferred ionic liquids freeze (that is to say they change fromtheir liquid state into their solid state) at a temperature which issimilar to the melting point. This can be readily determined by trialand error. Preferably, the decrease in impedance seen on melting isfully reversible with the impedance increasing to its initial high valueon re-freezing. A freezing point similar to the melting point isfavoured by a crystalline structure in the solid state.

Taking the above considerations into mind, the most preferred cationsare imidazolium and pyrrolidinium and substituted forms of thesecations. The most preferred anions are hexafluorophosphate andtris-(pentafluoroethyl) trifluorophosphate. A particularly preferredionic liquid is 1-methyl-tridecafluorooctyl-imidazoliumhexafluorophosphate, which has a melting point of about 80° C. Anotherparticularly preferred ionic liquid is1,1-dimethyl-pyrollidinium-tris-(pentafluoroethyl)-trifluorophosphate,which has a melting point of about 108° C.

Each end of the conductive sheath 10 is provided with a respective plug(not shown). Each plug seals its corresponding end of the sheath 10 soas to provide a fluid tight seal between the electrically conductivewire 12 and the electrically conductive sheath 10. The plugs (not shown)serve to retain the ionic liquid 14 within the conductive sheath 10. Theplugs are made entirely or partially out of a non-electricallyconductive material so that they do not provide an electricallyconductive path between the wire 12 and the sheath 10. Also provisionneeds to be made for electrical interrogation of the sheath/wireassembly.

The sheath 10 may be made of any suitable electrically conductivematerial. Preferably, the material also is a good conductor of heat.Typically, the sheath 10 will be made of a metal. A preferred metal isstainless steel which is resistant to oxidation and chemical corrosion.The wire 12 may also be made of any suitable electrically conductivematerial. Typically, the wire 12 will be a metal. A preferred metal iscorrosion resistant.

Preferably, the outside diameter of the sheath 10 is relatively small,for example 2 to 3 mm, so as to maximise the ratio of the surface areaof the sheath 10 to the internal volume. This helps to ensure that theionic liquid 14 rapidly attains the external temperature. Thetemperature switch may be any desired length (for example between 1 cmand 10 meters or more) with the sheath 10, the conductive wire 12 andthe ionic liquid 14 extending the full length of the temperature switch.

In order to assemble the temperature switch, the wire 12 is firstpositioned within the sheath 10 (with or without a porous spacingmaterial). One end of the sheath 10 is then immersed in the ionic liquid14 in its liquid state. A vacuum is then applied to the other end of thesheath 10 so as to draw the ionic liquid 14 into the sheath. After theionic liquid 14 has solidified the sheath is sealed with the two endplugs.

In use, the temperature switch may be used to protect, for example, amachine or structure which may suffer damage above a thresholdtemperature. One such structure is the wing of a aeroplane. In order toremove ice which has formed on the wing of an aircraft, or to preventthe formation of ice, hot air from the aircraft's engine can be directedto flow over the surface of the aircraft wing at risk. The hot air meltsor prevents the formation of ice. However, it is necessary to avoidheating the wing to too high a temperature, as this can damage the wing.Accordingly, the temperature switch described above may be used toprotect the wing. The temperature switch is used to provide a warningsignal when the temperature of the wing exceeds or is equal to athreshold temperature. This threshold temperature may be above thenormal operating temperature of the wing, but below (so as to provide asafety margin) the lowest temperature at which damage may occur.

The temperature switch can be attached to the wing to be protected. Itmay be, for example, configured in a straight line or in a serpentinearrangement on the heated surface of the wing. The ionic liquid 14 ofthe temperature switch has been chosen so as to have a melting pointcorresponding to the threshold temperature.

The electrically conductive sheath 10 and the electrically conductivewire 12 act as electrical contacts in contact with the ionic liquid 14.The electrically conductive sheath 10 and the electrically conductivewire 12 are connected to suitable electronics (of a known type) suitablefor measuring the conductivity or impedance of the ionic liquid 14between the electrically conductive sheath 10 and the electricallyconductive wire 12.

During normal operation, the temperature of the wing, and therefore thetemperature of the temperature switch, does not exceed the thresholdtemperature. Accordingly, the ionic liquid 14 remains in its solid stateand has a low conductivity (high impedance). However, should the wing beoverheated, such that the wing temperature equals or exceeds thethreshold temperature, then the ionic liquid 14 in the temperatureswitch will melt. At this point, the conductivity of the ionic liquid 14increases greatly (that is to say the impedance decreases). This isdetected by the electronics connected to the electrically conductivesheath 10 and the electrically conductive wire 12. On detecting theincrease in conductivity, the electronics produce a warning signal whichcan be used to alert the pilot and to divert the hot air from thesurface of the wing.

When the temperature of the wing has fallen back below the thresholdtemperature, the ionic liquid 14 solidifies and its conductivitydecreases accordingly, and its impedance thus increases to thepre-threshold value, i.e. the ionic liquid resets. Again, this can bedetected by the electronics.

As the ionic liquid is fully contained within the conductive sheath 10and the two end plugs (not shown), there is no loss of the ionic liquid14 while it is in its liquid state. Additionally, the ionic liquid ischosen such that its temperature stability is adequate for theapplication. This means that the temperature to which the ionic liquidis exposed does not reach the temperature at which the liquid undergoeschemical degeneration. Accordingly, once the ionic liquid has solidifiedthe temperature switch is undamaged and ready to protect the wing fromoverheating again.

There are known temperature switches similar to the exemplarytemperature switch described above and having a sheath and an innerwire. However, these known temperature switches use eutectic saltmixtures in place of the ionic liquid 14. The use of the ionic liquid 14in temperature switches of this type facilitates the provision of suchtemperature switches which have a relatively low threshold temperature.Most eutectic salt mixtures have a melting point above 200° C. Althoughthere are eutectic salt mixtures which have melting points below 200°C., these often prove problematic in temperature switches of the typedescribed above for a number of reasons. For example, they may have anundesirably low chemically stability, they may be moisture sensitive orthey may exhibit only a small change in conductivity on melting.Additionally, eutectic salt mixtures with melting points below 200° C.rarely demonstrate ideal impedance characteristics. On the other hand,ionic liquids having favourable characteristics and having meltingpoints below 200° C. can be readily identified. In fact, it is possibleto choose a suitable ionic liquid for almost any desired thresholdtemperature below 200° C.

It will be appreciated that the invention can be modified in a number ofways.

Instead of using the electrically conductive sheath 10 and theelectrically conductive wire as the electrical contacts which makecontact with the ionic liquid 14, any suitable electrical contacts maybe used. In the examples described above, the electrically conductivesheath 10 acts as a container to contain the ionic liquid 14. However,this need not be the case and any suitable container may be used tocontain the ionic liquid 14. The container need not be an electricalcontact.

For example, in another embodiment, a tubular conductive sheath housestwo conductive wires which are spaced from one another (i.e. nottouching) and which extend through the sheath. One of the wires iselectrically connected to the sheath while the other does not touch thesheath. An ionic liquid fills the sheath making contact with the sheathand with the two wires. When the ionic liquids melts, it becomes moreconductive and so the electrical impedance between the sheath and thewire which is not connected to the sheath greatly decreases. This isdetected by suitable electronics.

Alternatively, the two electrical contacts can be two electrical plateswith a gap between them. The gap is filled with an ionic liquid.

Instead of using a single ionic liquid—that is to say a single organiccation in combination with a single anion, a mixture of different ionicliquids may be used. A mixture of different ionic liquids may have adepressed melting point (as compared to the component ionic liquidsconsidered separately) and may have a reduced viscosity.

It will be appreciated that electronics may be used to measure anysuitable electric characteristic which undergoes a change on melting ofthe ionic liquid. For example, the electronics may measure conductivity,impedance or capacitance.

1. A temperature switch comprising a material which melts at apredetermined temperature and concomitantly undergoes a change in anelectrical characteristic at the predetermined temperature, and at leasttwo electrical contacts for detecting the change in the electricalcharacteristic, wherein the material comprises an organic cation and ananion; the cation is 1-methyl-tridecafluorooctyl-imidazolium; the changein the electrical characteristic occurs as the material melts; thepredetermined temperature is less than 200° C.; and the at least twoelectrical contacts comprise: an electrically conductive sheath and anelectrically conducting wire within and spaced from the sheath, thematerial lying between the sheath and the wire, and in contact with boththe sheath and the wire; two wires which extend, spaced from oneanother, through a sheath; or two plates spaced from one another.
 2. Atemperature switch comprising a material which melts at a predeterminedtemperature and concomitantly undergoes a change in an electricalcharacteristic at the predetermined temperature, and at least twoelectrical contacts for detecting the change in the electricalcharacteristic, wherein the material comprises an organic cation and ananion; the anion is tris-(pentafluoroethyl)-trifluorophosphate; thechange in the electrical characteristic occurs as the material melts;the predetermined temperature is less than 200° C.; and the at least twoelectrical contacts comprise: an electrically conductive sheath and anelectrically conducting wire within and spaced from the sheath, thematerial lying between the sheath and the wire, and in contact with boththe sheath and the wire; two wires which extend, spaced from oneanother, through a sheath; or two plates spaced from one another.
 3. Atemperature switch according to claim 1, wherein the material is1-methyl-tridecafluorooctyl-imidazolium hexafluorophosphate.
 4. Atemperature switch according to claim 2, wherein the material is1,1-dimethyl-pyrrolidinium-tris-(pentafluoroethyl)-trifluorophosphate.